The Mexico City track has the highest altitude of any circuit and by quite some margin, situated 2,285 metres above sea level. And that change in altitude has a surprisingly large effect on many elements of an F1 car.
While it's easily forgotten in the heat of a wheel-to-wheel battle or stunning pole position lap, the performance of a Formula One car and the ability for it to even run is largely down to the many, many invisible air particles floating around us.
Many of the F1 tracks that we visit are fairly close to sea level, and in Zandvoort's case very close, so the air density and altitude are pretty similar. But Mexico is an outlier. It has the highest altitude of any circuit and by quite some margin, situated 2,285 metres above sea level, five-times the height of the Petronas Twin Towers in Kuala Lumpur.
And that change in altitude has a surprisingly large effect on many elements of an F1 car, changing how it performs and operates...
The Autodromo Hermanos Rodriguez is located to the south-east of Mexico City's centre, with an ambient pressure of just 780hPa - normal sea level is around 1,000hPa, so it's roughly 20% less.
Despite its high altitude, it is one of F1's flattest tracks with an elevation change from the lowest to highest point of just under 3 metres. This is partly because Mexico City sits in the 'Valley of Mexico' on a former lake bed.
When it comes to the highest altitudes of other F1 tracks, none come close. In fact, the Mexico City venue is located nearly 1,500m higher than the next circuit on the list, which is Interlagos, measuring in at 800m above sea level.
So while the weather and temperatures experienced at the Mexico City Grand Prix aren't especially different to other race weekends, the atmospheric conditions are unique and provide teams with some rare challenges to tackle.
Altitude impacts everything that we do, whether it's going for a run around Mexico City or a turbocharger pumping oxygen into the engine of a car. And it's all related to the amount of air particles and the density of the air at that specific height.
The higher you are in the atmosphere, the thinner the air is. This is because air has weight and so the closer you are to sea level, the more the air is being compressed downwards, meaning denser air and more air particles. At 2,285 metres above sea level, there is around 25% less air density compared to at sea level and therefore a quarter less oxygen.
When you think about an F1 car, there are many crucial factors that ensure it operates correctly, three of which are: aerodynamics, cooling and the power unit. These elements are greatly impacted by the amount of air available to them and therefore, less air means differing performance.
High altitude doesn't directly impact the racing itself, because everyone is impacted in the same way and the long main straight and two DRS zones early in the lap do promote overtaking. However, different cars will be impacted by the effects of the altitude in different ways, some faring better and some faring worse, which can mix up the competitive order in Mexico.
Because of the thin air, the drag of a Formula One car in Mexico City is much lower. There are fewer air particles for the car to move out of the way, so the car cuts through the air quicker and with less disruption. This is why the cars are so fast on the straights in Mexico, with a maximum speed higher than Monza (350 km/h) whilst running wings as big as the ones we use in Monaco.
However, fewer air particles also have the impact of less downforce being generated, as there is less air pushing the car into the ground. In fact, the downforce loss is around 25% in Mexico because of the altitude. As a result, the highest downforce specification - Monaco level of wing - is used but this is generating the same level of downforce (or even slightly less) as the Monza wing because of the lack of air density.
Aero grip is therefore pretty low in Mexico, but you can run a big wing without the penalty of drag, so top speeds are very high.
In terms of the impact the altitude level has on the power unit, if we were talking about naturally aspirated engines, the performance difference at a high-altitude track would be much higher, as it relies on oxygen being drawn into the engine to complete the combustion process. This would produce a 25% performance loss, but on the modern-day F1 power units this is avoided thanks to the Turbocharger.
This is because the Turbo spins at an incredibly high speed to pump more air into the engine - around three times more air, in normal altitude conditions. More air means you can pump in more fuel and therefore generate more power. In Mexico, the Turbo has to work harder to compensate for the lower air density and it does this by spinning at a higher speed, in its attempt to make up some of the performance loss.
However, it can't make all of the performance difference up. Working the Turbo 20% harder just isn't possible - there isn't the margin left, because they are designed and built for normal race conditions, not the unique ambient pressure of the Autodromo. So, there is still a sizeable reduction in power unit output, but the lower drag helps make up for that and propel the cars to these incredible maximum speeds on the track's long main straight.
There is also less harvesting from the MGU-H in Mexico because less air into the engine means less power and less exhaust gasses for the MGU-H to recover and turn into useful energy. Some manufacturers will fare better than others depending on the size of their Turbo and the layout of their power unit system.
The way F1 cooling works is the cooler air particles pass through the cooling intakes, picking up the heat from the components before being dispersed out of the back of the car as hot air. A higher altitude means less air is passed through the radiators, air intakes and ducts which results in less cooling, meaning the various elements of the car such as the Power Unit and brakes run hotter or require much larger ducts to get things sufficiently cooled down.
Obviously, teams try and open up the car's cooling outlets as much as they can, increasing the size of the air intakes and ducts to bring more air particles in, but this also reduces the aerodynamic performance and increases the drag of the car, so a balance has to be found between the two.
Cooling the car appropriately is probably the biggest challenge in Mexico. For the power unit, the lack of mass flow of air limits the cooling potential, which requires careful management to ensure reliability. And overheating brakes can lead to accelerated wear or glazing (where the surface is burnt off, turns shiny and therefore drops friction). Furthermore, the turbo spinning at higher speeds causes additional mechanical strain on the turbine and compressor elements.
These are all delicate issues that teams have to consider, monitor and react to, which all add to the excitement and challenge of the Grand Prix.
Check out our Thursday gallery from Mexico City, here.
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